The present invention relates to heat-sensitive recording papers and more particularly, to such heat-sensitive recording papers having a well-improved recording aptitude for use in various heat-sensitive recording equipment including thermal printers, thermal recording type facsimile equipment or the like that can be obtained by adding calcined clay or calcined aluminum oxide into the color-developing layer of heat-sensitive recording papers.
Such heat-sensitive recording papers that utilize a heat color-developing reaction occurring between a colorless chromogenic dyestuff having a lactone, lactam, spiropyrane or the like structure and a phenol are disclosed in, for example, the Japanese Patent Publications No. 43-4160 and 45-14039 and the Japanese Laid-Open Patent Application No. 48-27736 and are now widely commercialized.
These heat-sensitive recording papers are produced by applying on the paper surface the coating prepared by individually grinding and dispersing a colorless chromogenic dyestuff and phenol into fine particles, mixing the resultant particles with each other and then adding thereto a binder, filler, sensitizer, slipping agent and other auxiliaries. When subjected to heat, the coating undergoe a chemical reaction which instantaneously develops a color, and various colors can be developed in clear shade depending upon the specific colorless chromogenic dyestuff selected.
These heat-sensitive recording papers has now been finding a wide range of applications, including medical or industrial recording instruments, terminals of electronic computer and date communication systems, printers of electronic desk calculators, facismile equipment, automatic ticket vending machines and so on.
These recording equipment has a heating element such as a thermal head or heating pen (stylus) and the heat-sensitive recording paper, when contacted with such an heating element, is heated thereby to develop a color for recording.
Thermal heads that have been produced so far are diverse in their materials used and configuration. Accordingly, since the requirements for heat-sensitive recording papers vary largely with the performance, controlling method and recording conditions (applicable voltage, pulse width, temperature, pressure, recording speed and contents of date to be recorded) of the specific thermal heads used, the matching scheme between the respective heat-sensitive recording papers and the latter is very important. Especially in recent years, as the applications of recording equipment tend to be diversified and call for a higher performance, a higher quality has come to be required for heat-sensitive recording papers to be used thereon.
Ordinarily, a thermal head is subjected to its heating and cooling cycles repeatedly at a short cyclic period of 0.5-20 milliseconds, and the color-developing layer of a heat-sensitive recording paper contacted with the thermal head receives heat energy generated by the latter to cause a color-developing reaction for recording purpose.
As a matter of course, the heat-sensitive recording paper must have a sufficient color-developing sensitivity for producing clear colored records with such a small heat imput from the thermal head. As the thermal head is heated and cooled repeatedly in the recording process, heat-sensitive materials contained in the color-developing layer of the heat-sensitive recording paper is once melted and then set repeatedly. In this course, a portion of the heat-sensitive materials may stick to the head surface and, consequently, the melted heat-sensitive materials may be accumulated on the head surface as residues. If this occures, such accumulated residues will obstructs the conduction of heat from the head to heat-sensitive recording paper to render printed images obscure and, sometimes, to such an extent that the records are hardly readable. Also, if the heat-sensitive paper adhers or sticks to the thermal head causing a so-called "sticking", the movement of the paper or head will be obstructed with generation of offensive sounds and, in the worst case, the recording function itself may become impossible.
Further, as the heat-sensitive materials deposited on the thermal head may be retransferred to the heat-sensitive paper surface, the thermal head may not be cooled sufficiently in the cooling cycle succeeding to the printing cycle due to the accumulation of the heat-sensitive material or an increase in the ambient temperature of the head may heat any portions of the heat-sensitive paper other than those required for intended printed images to cause undesired coloring thereat, the print image may be degraded with such phenomena as bleeding, smearing or ghost resulting therefrom.
To minimize such recording troubles as mentioned above and to improve recording aptitude of heat-sensitive papers, various additions are generally added to the coating to be applied on the color-developing layer of the heat-sensitive papers in addition to a chromogenic dyestuff, phenolic substance and binder constituting its basic ingredients.
For example, with a view to improving the color-developing sensitivity, preventing adhesion of the heat-sensitive materials onto the head and preventing a frictional pollution, such waxes as paraffin waxes, polyolefin waxes, fatty amides including their methylol derivatives, higher fatty acids and their metal salts, condensates of a higher fatty acid and amine, polyhydric alcohol esters of higher fatty acids, higher alcohols and so on are added to heat-sensitive coating formulations. Also, for improving the coating aptitude, whiteness and brightness or preventing adhesion of residues onto the thermal head, clays such as china clay, kaolin, talc, titanium oxide, calcium carbonate, magnesium carbonate, zinc oxide, starch and other fillers are used.
To meet the requirements for the heat-sensitive recording equipment such as thermal type printers and facsimile equipment showing a tendency towards a higher performance and speed, the aforementioned waxes must be added in large quantities. However, as the amount of such heat-meltable waxes is increased, occurence of troubles such as accumulation of residues on the thermal head bleeding, smearing or ghost can also be increased. In order to effectively prevent such troubles, clays and other fillers must be added in larger quantities to keep the color-producing agents (colorless chromogenic dyestuff and phenolic substance) dispersed in a separated state and to cause the color-developing agents and waxes heat-melted in recording to be absorbed by the fillers and stabilized therein in as much as possible. However, increasing the content of such fillers that do not directly contribute to the color-developing reaction will result in a decrease in the developed color density and an increase in the amount of coating to be applied. In this sense, it has been very difficult to prevent substantially perfecty the accumulation of residues and other troubles while securing an optimum developed color density.